Conventionally, an active matrix-type liquid crystal display device that includes a thin film transistor (TFT) as a switching element is known. The display unit of the active matrix-type liquid crystal display device includes a plurality of source bus lines (video signal lines), a plurality of gate bus lines (scanning signal lines), and a plurality of pixel formation portions that are respectively provided at intersections of the plurality of source bus lines and the plurality of gate bus lines. These pixel formation portions are arranged in a matrix form to configure a pixel array.
FIG. 41 is a circuit diagram showing a configuration of a pixel formation portion of a conventional general active matrix-type liquid crystal display device. As shown in FIG. 41, the pixel formation portion includes a thin film transistor T91 having a gate electrode connected to a gate bus line GL passing through a corresponding intersection and having a source electrode connected to a source bus line SL passing through the intersection, a pixel electrode 92 connected to a drain electrode of the thin film transistor T91, a common electrode (counter electrode) COM and an auxiliary capacitance electrode CS provided so as to be shared among the plurality of pixel formation portions, a liquid crystal capacitor Clc formed by the pixel electrode 92 and the common electrode COM, and an auxiliary capacitor Cstg formed by the pixel electrode 92 and the auxiliary capacitance electrode CS. Further, a pixel capacitance is formed by the liquid crystal capacitor Clc and the auxiliary capacitor Cstg. Then, a voltage indicating a pixel value is held in the pixel capacitance, based on a video signal that a source electrode of the thin film transistor T91 receives from the source bus line SL when a gate electrode of the thin film transistor T91 receives an active scanning signal from the gate bus line GL. Note that the auxiliary capacitor Cstg is not necessarily provided.
Further, a liquid crystal display device having a configuration in which one pixel is divided into a plurality of (typically, two) sub pixels to improve the view angle characteristic is also known (for example, refer to JP 2006-133577 A). This configuration is referred to as a “multi-pixel structure”. In the liquid crystal display device having the multi-pixel structure, a liquid crystal is driven so that brightnesses of the plurality of sub pixels become mutually different brightnesses. FIG. 42 is a circuit diagram showing a configuration example of a pixel formation portion in the conventional liquid crystal display device having the multi-pixel structure. As shown in FIG. 42, in this liquid crystal display device, a pixel formation portion PIX9 is constituted by two sub pixel units (a first sub pixel unit PIX9a and a second sub pixel unit PIX9b). Both sub pixel units (PIX9a, PIX9b) include transistors (T92, T93), pixel electrodes (E91, E92), liquid crystal capacitors (ClcA, ClcB), and holding capacitors (CstA, CstB), as common constituent elements. The second sub pixel unit PIX9b further includes a transistor T94 having a gate electrode connected to a scanning signal line GLi+1 and having a source electrode connected to the pixel electrode E92, a capacitance electrode E93 connected to a drain electrode of the transistor T94, and a buffer capacitance Cdown formed by the capacitance electrode E93 and a common electrode (auxiliary capacitance electrode) COM 102. In such a configuration, when the scanning signal line GLi is in a selected state, the potential of the pixel electrode E91 in the first sub pixel unit PIX9a becomes equal to the potential of the pixel electrode E92 in the second sub pixel unit PIX9b. Thereafter, when the scanning signal line GLi+1 is in the selected state, the transistor T94 is placed in an on state. Accordingly, a charge moves between the pixel electrode E92 and the capacitance electrode E93, and the potential of the pixel electrode E92 varies. As a result, the pixel electrode E91 and the pixel electrode E92 have different potentials, and the first sub pixel unit PIX9a and the second sub pixel unit PIX9b have different brightnesses.
In recent years, development of high definition of a display image in the liquid crystal display device is remarkable. Examples of high definition include 4K (resolution: 3840×2048) of a television large panel. When the display image has high definition, power consumption associated with the drive of the panel increases. Regarding the power consumption of the panel, power due to charge and discharge of a source bus line is the majority. The power consumption due to the charge and discharge of the source bus line is obtained from (the number of source bus lines)×(wiring capacitance of source bus lines)×(drive frequency)×(square of amplitude of video signal). Therefore, by setting the amplitude of the video signal small, the power consumption of the panel can be effectively reduced. JP 2009-109600 A discloses the invention of a liquid crystal display device which enables reduction of the amplitude of the video signal by amplifying the pixel electrode potential. In this liquid crystal display device, the pixel formation portion is configured as shown in FIG. 43 so that the following drive is performed. In the first half of one horizontal scanning period, an on-level potential is applied to a line denoted by a reference character 9 in the state where an off-level potential is applied to the gate bus line GL. Accordingly, thin film transistors T902, T903 are placed in the on state. As a result, a video signal potential (a potential of the source bus line SL) is applied to a node 901, and a potential of the common electrode COM is applied to a node 902. Thereafter, in the latter half of the one horizontal scanning period, the on-level potential is applied to the gate bus line GL in the state where the off-level potential is applied to the line denoted by the reference character 9. Accordingly, the thin film transistor T901 is placed in the on state. As a result, a video signal potential is applied to the node 902. That is, the potential of the node 902 rises from the common electrode potential to the video signal potential. At this time, because the node 901 is in the floating state, the potential of the node 901 rises via a capacitance C91 with the rise in the potential of the node 902. As described above, a larger voltage is applied between the pixel electrode and the common electrode.